272 | Thermodynamics
find the simplest expression for the mixture temperature writ-
ten in the form
T 3 fam#
1
m# 3,m#
2
m# 3, T 1 , T 2 bbuilding is located in San Francisco, California, where the
average winter temperature is 12.2°C, and is maintained at
22°C at all times. The building is heated by electricity whose
unit cost is $0.09/kWh. Determine the amount and cost of the
heat “vented out” per month in winter.
5–164 Consider a large classroom on a hot summer day
with 150 students, each dissipating 60 W of sensible heat. All
the lights, with 6.0 kW of rated power, are kept on. The room
has no external walls, and thus heat gain through the walls
and the roof is negligible. Chilled air is available at 15°C,
and the temperature of the return air is not to exceed 25°C.
Determine the required flow rate of air, in kg/s, that needs to
be supplied to the room to keep the average temperature of
the room constant. Answer:1.49 kg/s
5–165 Chickens with an average mass of 2.2 kg and aver-
age specific heat of 3.54 kJ/kg · °C are to be cooled by
chilled water that enters a continuous-flow-type immersion
chiller at 0.5°C. Chickens are dropped into the chiller at a
uniform temperature of 15°C at a rate of 500 chickens per
hour and are cooled to an average temperature of 3°C before
they are taken out. The chiller gains heat from the surround-
ings at a rate of 200 kJ/h. Determine (a) the rate of heat
removal from the chickens, in kW, and (b) the mass flow rate
of water, in kg/s, if the temperature rise of water is not to
exceed 2°C.
5–166 Repeat Prob. 5–165 assuming heat gain of the chiller
is negligible.
5–167 In a dairy plant, milk at 4°C is pasteurized continu-
ously at 72°C at a rate of 12 L/s for 24 h a day and 365 days
a year. The milk is heated to the pasteurizing temperature by
hot water heated in a natural-gas-fired boiler that has an effi-
ciency of 82 percent. The pasteurized milk is then cooled by
cold water at 18°C before it is finally refrigerated back to
4°C. To save energy and money, the plant installs a regenera-
tor that has an effectiveness of 82 percent. If the cost of nat-
ural gas is $1.10/therm (1 therm 105,500 kJ), determine
how much energy and money the regenerator will save this
company per year.m 1 , T 1
Mixing device- m• 2 , T 2 m^3 , T^3
FIGURE P5–1605–168E A refrigeration system is being designed to cool
eggs (r67.4 lbm/ft^3 and cp0.80 Btu/lbm · °F) with an
average mass of 0.14 lbm from an initial temperature of 90°F12.2°CBathroom
22 °CFan30 L/sFIGURE P5–16372 °C72 °C
Hot milkRegenerator Cold
milk4 °C
Heat
(Pasteurizing
section)FIGURE P5–1675–161 An ideal gas expands in an adiabatic turbine from
1200 K, 600 kPa to 700 K. Determine the turbine inlet vol-
ume flow rate of the gas, in m^3 /s, required to produce turbine
work output at the rate of 200 kW. The average values of the
specific heats for this gas over the temperature range are cp
1.13 kJ/kg · K and cv0.83 kJ/kg · K. R0.30 kJ/kg · K.
5–162 Consider two identical buildings: one in Los Ange-
les, California, where the atmospheric pressure is 101 kPa
and the other in Denver, Colorado, where the atmospheric
pressure is 83 kPa. Both buildings are maintained at 21°C,
and the infiltration rate for both buildings is 1.2 air changes
per hour (ACH). That is, the entire air in the building is
replaced completely by the outdoor air 1.2 times per hour on
a day when the outdoor temperature at both locations is
10°C. Disregarding latent heat, determine the ratio of the heat
losses by infiltration at the two cities.
5–163 The ventilating fan of the bathroom of a building has
a volume flow rate of 30 L/s and runs continuously. The